Chiral-nematic, also known as cholesteric, liquid-crystalline materials are useful in a variety of applications including various LC display components, reflective films, optical filters, polarizers, paints, and inks, among others. Methods for preparing such materials are well established. See, e.g., Giovanni Gottarelli and Gian P. Spada, Mol. Cryst. Liq. Crys., Vol. 123, pp. 377–388 (1985); Gian Piero Spada and Gloria Proni, Enantiomer, Vol. 3, pp. 301–314 (1998). However, improvement is still needed. While early uses of chiral-nematic compositions relied upon mixtures composed mostly of chiral components, more recently such materials are composed of nematic LC mixtures combined with small amounts of chiral dopants. In such new compositions the properties of the nematic host material, for example viscosity, birefringence, electrical anisotropy, and magnetic anisotropy among others, are tailored to the desired usage by altering the chemical composition of the nematic mixture, and then a chiral dopant is incorporated to induce helical twisting so as to provide the desire chiral-nematic pitch. It is apparent that the properties of this chiral nematic composition are therefore a combination of the properties of the nematic host plus those of the dopant. It is further well understood that by reducing the amount of dopant, the properties of the host nematic LC formulation might be better preserved. Certainly, reducing the concentration of a specific dopant also reduces the pitch of the resulting chiral-nematic formulation. Many uses of chiral-nematic compositions require the formulation to reflect or transmit visible light, thus requiring compositions with substantial helical twist, i.e. short helical pitch (“p”). These considerations indicate that dopants that induce large amounts of nematic helical twist per unit concentration are prized. The figure of merit for such materials is its Helical Twisting Power (“HTP” or β).
A dopant material's HTP (β) is defined, in a specified host at a particular temperature, by Equation 1:β=(pcr)−1  (Equation 1)wherein the “p” is the measured helical pitch of the doped nematic (μm); “c” is the measure of the dopant concentration (usually in terms of mole fraction, weight fraction, or weight percent on a unitless scale, wherein mole fraction and weight fraction is on a scale of 0 to 1); and “r” is the enantiomeric excess of the dopant (on a unitless scale of 0 to 1). Enantiomeric excess (r) is defined as the absolute value of the difference in mole fraction (F) of the two enantiomer in a sample r equals |F(+)−F(−)|. Thus, for a racemic mixture r equals |0.5−0.5|=0; for an enantiomerically pure material r equals |1.0−0|=1; and for a 75% pure mixture the r equals |0.75−0.25|=0.5. The larger the HTP the lower the concentration of dopant needed to provide a specific pitch, and thereby yield a particular reflectance or transmission. The pitch of a chiral-nematic formulation can be measured using a variety of optical techniques. For example, see Zvonimir Dogic and Seth Fraden, Langmuir, Vol. 16, pp. 7820–7824 (2000). The dopant concentration is as formulated and the enantiomeric excess can be measured via chiral high-performance liquid chromatography (HPLC) or nuclear magnetic resonance (NMR) spectroscopy. Typically, for useful enantiomerically pure dopants, their HTP's range from one to several hundred (μm−1). Dopants with twisting power greater than 100 (based on dopant mole fraction) are often described as “high twist” dopants. The discovery of new dopants, particularly high twist dopants, is important to broadening the utility of chiral-nematic formulations.
Not only can chiral-nematic liquid crystals be formulated to reflect various wavelengths of incident electromagnetic radiation, but it is well understood that reflected light is circularly polarized, depending upon the sense of chirality of the helical pitch. Thus, a chiral nematic displaying a right-handed helical mesostructure will reflect right-handed incident light. For many applications it is useful to be able to reflect both right-handed and left-handed senses of circularly polarized light, for example, in a vertically layered structure. It is further well known that enantiomers of a chiral-dopant structure induce the opposite polarity of helical rotation and, therefore, afford oppositely polarized light reflections. For this reason the preparation of enantiomeric pairs of dopants for use in separate light modulating layers can be particularly useful.
There are three general sources for obtaining substantially enantiomerically pure organic compounds for use as dopants or more likely as synthetic precursors for dopants: (1) compounds available from natural sources; (2) the preparative separation of racemic mixtures of enantiomers; or (3) chiral synthetic methods that directly afford desired enantiomers. Most commonly, only the latter two methods provide access to both enantiomers of a potential dopant. Natural sources generally provide only one of any enantiomeric pair, reflecting the fundamental chirality of life. Thus, using natural sources for dopants or their precursors can lead to limitations in dopant utility. A discovery of new dopants available from non-natural sources would therefore be especially useful.